Background: Sun-induced skin damage, which increases skin cancer risk, is initiated in early life and promoted through later sun exposure patterns. If sun safety determinants are well understood and addressed during the school years, skin cancer incidence might be reduced. This study tested psychosocial influences on youth's sun safety and assessed their strength within and across gender and ethnicity in a sample of 1782 middle school students. Methods: Predictors included sunburn and skin cancer knowledge, tanning attitudes, peer norms, and barriers regarding sun exposure and were assessed with a self-administered, validated questionnaire. The hypothesized relationships were tested with structural equation models and confirmed with multilevel regression. Results: Across gender and ethnicity, knowledge emerged as an important sun safety predictor with both direct and indirect effects mediated through tanning attitudes. The relationship with barriers did not reach statistical significance within any of the subgroups, possibly due to measurement limitations. An indirect effect of peer norms on sun safety, mediated through tanning attitudes, was confirmed only among girls. Also, an indication that peer norms operate differently within the ethnic groups was found, since this predictor had a statistically significantly stronger relationship with sun safety among non-Hispanics. Conclusions: Youth's sun safety is a multifactorial practice, partially determined by ethnicity- and gender-based standards. In order to ensure health-promoting school environments, needed are multicomponent programs where peer norms and knowledge are salient and where sun safety is addressed individually and together with other health risk behaviors. (Contains 4 figures and 2 tables.)